1. Field of the Invention
The present invention relates to pumps which deliver a preselected volume of fluid for each pump stroke and, more particularly, relates to sampling pumps of the type commonly used to intermittently withdraw sample hydrocarbon fluids from a pipeline and input those fluids to a sample vessel for subsequent analysis.
2. Description of the Background
Sampling pumps have been used for years to intermittently withdraw fluid samples from a hydrocarbon pipeline and input those samples to a suitable container for subsequent analysis. The price charged for hydrocarbons being transmitted through the pipeline is typically determined as a function of (a) the volume of the actual fluid transferred through the pipeline, and (b) the BTU content of sample fluid intermittently withdrawn from the pipeline. The sample fluid vessel is periodically transported to a laboratory for fluid analysis by any suitable instrument, such as a gas chromatograph. The accuracy of the sampling technique, i.e., the caloric or BTU value of the fluid sample in the vessel compared to the average BTU value of all the fluid transmitted through that pipeline, thus has a significant affect on the price paid for the transmitted fluid. Accordingly numerous attempts have been made and will continue to increase the reliability and accuracy of this sampling technique, while at the same time lowering the overall sampling costs.
One version of a sampling pump is disclosed in U.S. Pat. No. 4,403,518. U.S. Pat. No. 4,470,773 discloses a similar pump with a feature for breaking the vacuum in the pump which could otherwise be created by the retraction of the collection probe from the resilient plug. The prime mover for reciprocating the sampling pump piston is typically a diaphragm, which in turn is powered by a pressurized air source, as disclosed in U.S. Pat. Nos. 4,440,032 and 4,525,127. U.S. Pat. No. 4,557,157 discloses a sampling pump which also utilizes this type of prime mover, and further discloses a pressure balanced check valve external of the pump body. This check valve utilizes line pressure as a reference, and enables the pump to perform its desired function of transmitting a preselected fluid volume per stroke regardless of line pressure.
Most sampling pumps are primarily intended to pump either a liquid or a gas, but not both. Generally, gas sampling pumps typically transfer up to 0.04 cc of gas to the sample vessel per pump stroke, while liquid sampling pumps typically operate in the range of from 0.5 cc to 3 cc of fluid per pump stroke. Since the gas sampling pump takes "small bites," precise displacement of the pump piston must be controlled. A pump intended for sampling a typical substantially incompressible liquid, on the other hand, is usually provided with a large diameter pump piston, so that the stroke of the piston and thus the size of the pump may be maintained within a practical range. A representative liquid sampling pump is disclosed in U.S. Pat. No. 4,172,670, while a gas sampling pump is described in U.S. Pat. No. 4,531,895. Sampling pumps designed to pump gases are promoted as having little or no pump "dead volume" which adversely affects sampling reliability. All the gas drawn into the sampling pump during each stroke is transferred from the pump to the sample vessel, and no substantial amount of gas remains in the pump body when the piston is in its most downward position adjacent to the pump inlet. A combined pump and sample vessel is disclosed in U.S. Pat. No. 4,628,750.
U.S. Pat. No. 4,531,895 referenced above discloses a sampling pump which again utilizes a diaphragm as the driver for the pump piston, although the balanced check valve concept is obtained by a seal, valve, and valve seat each radially within of the sampling pump body. This patent also teaches a cylindrical plunger and a disc inlet valve, each having a snug fit within the cylindrical piston-receiving bore in the pump body. Due to reduced material and manufacturing costs, a sampling pump with an internal balanced check valve as disclosed in the '895 patent is generally preferred over a pump with a balanced check valve mechanism external of the pump body.
The sampling pump manufactured according to the '895 patent has several disadvantages which have limited its acceptance in the industry. In order to keep the size and cost of the pump within acceptable limits, the piston is relatively small, in part because the balanced check valve mechanism is positioned radially between the piston and the pump body. Accordingly the pump is primarily intended as a gas sampling pump, and is generally not considered acceptable for pumping the significantly larger fluid volume per pump stroke which is customarily required for sampling liquids.
In the design of the sampling pump as disclosed in the '895 patent, the balanced check valve seal is stroked by the piston during its downward and upward stroke, thereby wearing the seal. Moreover, the return or upstroke of the piston tends to unseat this check valve seal, which under certain conditions may allow fluid collected in the sample vessel to flow back into the pump, thereby destroying the integrity of the sample. Also, the gas inlet seal according to this design may lift off its seat during the pump return stroke, which then decreases the life and reliability of this seal.
Users of sampling pumps are justifiably concerned that filtering devices between the process line and the sampling pump may alter the composition of sampled fluid compared to the process line fluid. A pressure drop across such a filter provided in the "hot loop" of fluid upstream from the pump inlet port may adversely affect sampling reliability. An effective hot-loop filter would encounter significant plugging problems, and thus would significantly increase maintenance costs. Due to these concerns, process line fluid is typically not filtered, and particles in the process line which pass into the sampling pump have long had an adverse affect on pump reliability.
The operator for reciprocating a sampling pump piston must be sized sufficiently large to stroke the piston during its power stroke, thereby expelling sample fluid from the pump and into the sample vessel. The pressure of the fluid in the sample vessel may either be nominal or considerable, and may be less than or greater than the line pressure of the hydrocarbons or other fluid being transmitted through the pipeline. The cost of a large operator for stroking the sampling pump piston represents a significant factor in the manufacturing costs of the sampling pump, while the size of the required operator may result in significantly increased installation and repair costs.
Certain types of injection pumps are structurally and operationally similar to a sampling pump. The present invention thus also relates to injection pumps of the type utilized to inject a specific quantity of low pressure fluid into a high pressure pipeline. In an exemplary use of an injection pump, an oderizer is input to a high pressure natural gas line for leak detection. As is the case for sampling pumps, both the quantity of fluid pumped per stroke and the time interval between successive pump strokes is selectively adjustable. Although the pump according to present invention is primarily described hereafter as a sampling pump useful for sampling purposes previously described, it should be understood that the same pump may be referred to as an injection pump when installed in a system for fluid injection purposes.
The disadvantages of the prior art are overcome by the present invention, and an improved sampling pump is hereinafter disclosed. The pump of the present invention is suitable for reliably withdrawing various sample hydrocarbon fluids from a pipeline at various line pressures and inputting those fluids to a desired sample vessel.